Understanding the principle

Light and its behaviour and properties
Ytre
Posts: 7
Joined: Fri Oct 16, 2020 5:49 am

Re: Understanding the principle

Post by Ytre »

I agree that fringe system is 3D, but I meant the intersection of two fronts. I see it as hyperbola.
I didn’t understand why it is impossible to demonstrate a hologram in real time? Due to air fluctuations or it is principally impossible? You didnt explain.
Din
Posts: 402
Joined: Thu Mar 12, 2015 4:47 pm

Re: Understanding the principle

Post by Din »

It depends on what you mean by "real time". If you set up a real object, illuminate it with a laser, and let the laser light hit a white card, then you put a reference beam on the white card, you will not see interference lines on the white card because they're too small. Usually, the number of interference lines are denoted by the spatial frequency, and, typically, spatial frequencies are in the order of 1000 lines/mm.

You can see a "real time" hologram if you make a hologram in the traditional way, replace the hologram, with the object still in place, reconstruct the hologram and slightly disturb the object. Now, the wavefront from the original hologram and the wavefront from the reconstructed hologram are slightly out of phase, and you get a beat frequency.
Ytre
Posts: 7
Joined: Fri Oct 16, 2020 5:49 am

Re: Understanding the principle

Post by Ytre »

I dont need to see interference lines. I wanna know of possibility making holographic display as shown on "avatar". I think we could make holographic "object" in space if behaviors of the film and empty space in a place were film is supposed to be are simillar. Is is it so or not?
lobaz
Posts: 280
Joined: Mon Jan 12, 2015 6:08 am
Location: Pilsen, Czech Republic

Re: Understanding the principle

Post by lobaz »

If you mean the display with the terrain from the film Avatar (https://james-camerons-avatar.fandom.co ... ons_Center), this is hardly possible with classical holography. If fact, illusions floating in the air above an table are still possible just in sci-fi. I am not aware of any pysical principle that could potentially make it.
Simple explanation: when you see a ball, light ray from a ball must hit your eye. When you want an illusion of a ball, the light ray still must originate somewhere. As a light ray cannot turn on its own, it follows that the light source ("a display") must be in line with your eye and the ball illusion. Thus, it is possible to make an illusion floating in front of some display, say a conventional hologram. But if you want to watch such an illusion "from side", it disappears as a light ray starting on the dispay surface have no way to reach both the illusion AND your eye.
Moreover: if you find a way how to bend a ray at a given point in the air (such as projection into fog), you have no way to block other light from passing that point. This means that the illusion is transparent, it cannot cover neither other parts of the illusion nor any real object.
Din
Posts: 402
Joined: Thu Mar 12, 2015 4:47 pm

Re: Understanding the principle

Post by Din »

There is a way of fooling the eye. If you have a horizontal disc containing a hologram, and illuminate it from below so that the image appears above the disc, it fools the eye to think that the object sits in free space. Of course, as Petr says, your vision is limited to the area of the disc, but most people seeing the illusion aren't aware of this. It isn't real time, you have to pre-record the hologram. But, so many people started to call it the "Leia" illusion, that we just ended up calling this our :Leia display.

https://www.youtube.com/watch?v=zgcKKgQPjuw
https://www.youtube.com/watch?v=1Nq4ri2t-HI
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